The present invention relates to the measurement of trace elements in a gas, particularly to the measurement of trace elements in He carrier gas of a hand-held gas chromatograph, and more particularly to a glow discharge detector using a floating pseudo-electrode for measuring trace elements of the He carrier gas, the glow discharge being controlled through a biased resistor.
In recent years, efforts have been directed to the development of a hand-held (portable) gas chromatograph. In a separation column of a gas chromatograph, chemical trace elements are carried by a carrier gas and each separated into small gas plugs through interaction with a coating inside the column. There has been a need for a highly sensitive detector for the detection of these trace chemical elements in these small gas plugs passing through the column for a gas chromatograph (GC). Among all conventional detectors used in the commercial GC systems, ion cell detectors seem to be physically most suitable for a portable GC system. However, in these conventional ion cell detectors, electrons and ions are generated by means of radioactive elements. Radioactive elements are hazardous and not suitable for general applications.
In normal ion cells, ions are generated by either radioactive isotopes, such as nickel 63, or pulsed arc sources. The percentage of ions of the trace elements generated is dependent upon their operational modes. In an electron capture mode of operation, almost all the trace element molecules are ionized by capturing electrons. However, the mode of operation is limited to the cases where trace element molecules have electron negativity.
In an ionization mode of operation, only a small portion of trace element molecules are ionized. The total amount of trace element ions is certainly dependent upon the available sample volume. In a portable GC, the available sample volume is generally quite small, in the order of micro-liters. For low concentration trace elements, the signal generated through direct electron measurement is quite small and may well be below the sensitivity of existing electron instruments. Except for more costly optical detection methods, all the prior ion cells are not applicable to small sample size in the portable GC for general applications.
Recent progress in micro-machining technology has enabled the development of miniaturized gas chromatography (GC) systems with micromachined fluidics and detectors, as exemplified by U.S. Pat. No. 5,583,281 issued Dec. 10, 1996 to C. M Yu. However, the sensitivity of these early micro GC systems was well below that of conventional systems due to limitation of the micro thermal conductivity detectors (TCD) used in most of the micro GS systems. Thus, efforts were directed to the development of a highly sensitive micro GC detector, which could replace the TDC in most portable GC systems and which has a potential to outperform some conventional GC detectors, such as the commonly used flame ionization detector (FID), nitrogen-phosphorous detector (NPD), and electron capture detector (ECD). These prior conventionally used high performance GC detectors are also sophisticated, heavy, large, and require either make-up and detector gases and/or radioactive materials to operate, and they are not suitable for field applications where portability is a top priority.
A TCD, on the other hand, employs thermal conductivity differences in various gas species to sense the change in gas composition. Although a TCD lacks sensitivity when compared with a FID, NPD and ECD, it is a much simpler detector and is much easier to be adapted for field use. Such are exemplified in U.S. Pat. No. 5,591,896 issued Jan. 7, 1997, to G. Lin, and in P. Dai et al, A Novel High Sensitivity Micro GC Detector, Transducers 99, Jun. 7-10, 1999, pp. 696-699, Sandai, Japan. These sensors have two electrodes mounted along a single axis on a base substrate, and the two electrodes are separated by a narrow gap. One of the electrodes is tapered into a fine apex to create a strong concentration of electric field around the apex. When electric potential imposed upon the electrodes is sufficiently high, the gas molecules around the apex will be ionized. The ions and electrons generated by the ionization create an electric current flowing between the two electrodes across the gap. The electric current changes when gas composition changes because different gas molecules have different molecular structure and consequently different ionization characteristics, and this change is used as the micro detector""s sensing signal. Polarity of the micro sensor can be set with the tapered electrode as either cathode or anode. These detectors measure the ionization properties of the sample gas in the glow discharge.
Recently, Hewlett Packard has developed a xe2x80x9cCapillary Electron Capture Detector (ECD)xe2x80x9d which also utilizes a pair of electrodes, one being tapered and one being hollow. This detector measures the effect of electron capture by means of sample gas molecules.
In a portable GC, an ion cell has to be non-radioactive, low power, low noise, and low cost, but rigid with high detection sensitivity.
The present invention provides a solution to the above-mentioned problem by providing a highly sensitive electronic ion cell which utilizes direct current (DC) glow discharge for the measurement of trace elements, in a carrier gas, such as He. The glow discharge detector of this invention involves a constant wave (CW) direct current glow discharge controlled through a biased resistor. The glow discharge detector utilizes an extra floating pseudo-electrode to form a capacitor at the cathode dark space to detect the trace elements. The voltage drop between the cathode and the pseudo-electrode varies due to trace amounts of chemical components.
It is an object of the present invention to provide a means for measuring trace elements in a carrier gas.
A further object of the invention is to provide a glow discharge detector for measuring variations of electron density due to trace amounts of chemical components in a carrier gas.
Another object of the invention is to provide a glow discharge detector for measuring trace elements in an He carrier gas of a gas chromatograph.
Another object of the invention is to provide a direct current, constant wave glow discharge detector.
Another object of the invention is to provide a glow discharge detector controlled through a biased resistor and provided with a floating pseudo-electrode forming a probe.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The present invention involves a glow discharge detector particularly applicable for the measurement of trace elements in He carrier gas of a portable (hand-held) gas chromatograph. The glow discharge detector is of a direct current (DC), constant wave (CW) type and utilizes a floating pseudo-electrode to form a probe in the plasma. The probe enables direct measurement of the large variation of cathode drop voltage due to trace amounts of chemical components in the He carrier gas, which is many orders of magnitude larger than that caused by direct ionization or electron capture.